In an RFID system resonant circuits are generally used in both the reader and the transponder. Their use increases the efficiency of energy transfer between the two circuits, which would otherwise be much lower, severely limiting the range of operation. Optimal read range may be achieved when the reader is stimulated at its resonance frequency, and this also matches the resonant frequency of the transponder. The task of operating both these units at their resonant frequencies is complicated by the following factors:                1) The LC manufactured components have tolerances on their values, and the resonant frequency will vary between different examples of nominally identical resonant systems.        2) Temperature can change the value of the components, making the resonance frequency drift over time and environment.        3) Metallic or magnetic objects placed in the vicinity of either antenna can change their inductance and therefore change the resonant frequency.        4) The desired resonant frequency can change, for example the regulatory frequency band can change across national boundaries.        
The impact of these complications increases with the Q of the two resonances. As the Q increases then the resonance bandwidth drops proportionally and a closer match between the two frequencies is required for efficient power transfer. Such constraints may limit the Q of the two circuits to relatively low levels, which may in turn limit the read range and/or lead to excessive power requirements.
In RFID systems one approach to mitigate some of these complications is to use a tuning circuit. A large number of different tuning variants have been disclosed in the prior art, however they have some common features. In order to tune the resonance frequency then either an electrically tunable component, such as a varactor or electrically variable inductor, or more commonly a combination of discrete capacitors or inductors are coupled into the resonance. An exemplary reference is U.S. Pat. No. 6,317,027, where a set of tuning capacitors in binary weighting are selectively coupled into the reader resonance with corresponding variations in the resonance frequency.
The adjustment may be carried out in a tuning cycle separate from the normal read and/or write operations. U.S. Pat. No. 6,317,027 is an example of such an approach, where a sweep of the tuning capacitance is carried out and the resonance amplitude profile determined; the chosen tuning capacitance achieves the maximum amplitude in the reader antenna. Alternatively U.S. Pat. No. 5,491,715 discloses a method based on the phase difference between the stimulus and the resonance to determine how far from resonance the reader circuit is.
These prior art methods to tune a system to resonance at a desired frequency have some significant drawbacks, which are now outlined.
The tuning circuit can require many components, comprising the set of capacitors and the switches to couple them into the resonance; this adds to the system complexity and cost. If fine control over the frequency is required then the selectable capacitors are required at high accuracy in order to achieve a monotonic setting of the total tuning capacitance. Such constraints can limit the feasible tuning accuracy and resolution.
The tuning is not generally real-time. If it is made real-time, such as the method described in U.S. Pat. No. 5,491,715, then this adds complexity and cost. Real-time control is beneficial to cope with transient de-tuning effects such as temperature drift and metallic objects coming into range.
These prior art tuning methods are generally applied to the reader antenna only. A transponder is required to have its natural resonance close to the radio frequency energising field in order to obtain its power. If significantly de-tuned, then the transponder would not be able to charge up enough to run a tuning circuit. As such, the prior art tuning methods described are usually unsuitable for a passive transponder.
Because of tight cost and power constraints it is more common that transponders are set up at manufacture with a fixed tuning to their target operating. This fixed tuning step adds to the cost of manufacture and cannot adjust to environmental changes that might affect the transponder resonant frequency.
A further transponder consideration is that it may be advantageous for a single transponder to be able to respond to a range of frequencies. One application example is the use of the same transponder across borders with different regulatory operating bands. If the transponders may respond to the different frequencies permitted in each region without a re-tuning step then this facilitates international travel, a key requirement in the common RFID application of asset tracking. Prior art methods provide no facility for this.
Further prior art may be found in U.S. Pat. No. 2,774,060, U.S. Pat. No. 3,818,472, U.S. Pat. No. 6,476,708, and GB2278631, WO2005/104022, US2004/0214549, GB2321726, U.S. Pat. No. 3,842,246, GB2087564, GB1206925.